Light emitting device

ABSTRACT

The luminance of different colors of light emitted from EL elements in a pixel portion of a light emitting device is equalized and the luminance of light emitted from the EL elements is raised. The pixel portion of the light emitting device has EL elements whose EL layers contain triplet compounds and EL elements whose EL layers contain singlet compounds in combination. The luminance of light emitted from the plural EL elements is thus equalized. Furthermore, a hole transporting layer has a laminate structure to thereby cause the EL elements to emit light of higher luminance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device having an element in which aluminous material is placed between electrodes (hereinafter referred toas light emitting element) (the device will hereafter be called a lightemitting device). Specifically, the invention relates to a lightemitting device having a light emitting element that employs as theluminous material an organic compound capable of providing EL (electroluminescence) (hereinafter referred to as EL element).

2. Description of the Related Art

In recent years, researches have been advanced on an EL element having astructure in which a thin film formed of an organic compound capable ofproviding EL (EL layer) is placed between an anode and a cathode, andlight emitting devices utilizing the luminous characteristic of the ELelement have been developed.

An EL layer usually has a laminate structure typical example of which isone proposed by Tang et al. of Eastman Kodak Company and composed of ahole transporting layer, a light emitting layer, and an electrontransporting layer. This structure has so high a light emittingefficiency that it is employed in almost all of EL displays that areunder development at present.

Other examples of the laminate structure of the EL layer include astructure in which a hole injection layer, a hole transporting layer, alight emitting layer, and an electron transporting layer are layered onan anode in this order, and a structure in which a hole injection layer,a hole transporting layer, a light emitting layer, an electrontransporting layer, and an electron injection layer are layered on ananode in this order. The light emitting layer may be doped with afluorescent pigment or the like.

In this specification all the layers that are placed between an anodeand a cathode are collectively called an EL layer. Therefore the holeinjection layer, the hole transporting layer, the light emitting layer,the electron transporting layer, and the electron injection layermentioned above are all included in the EL layer.

When a given voltage is applied to the EL layer structured as above by apair of electrodes, recombination of carriers takes place in the lightemitting layer to emit light. A light emitting element composed of ananode, an EL layer, and a cathode is called herein an EL element.

In an EL element, degradation of its EL layer is accelerated when adriving voltage is high. Therefore an organic compound emitting Light bya triplet exciton (hereinafter referred to as triplet compound) issometimes used instead of the usual luminous material, namely, a singletcompound (an organic compound that emits light by singlet exciton),because the triplet compound can emit light of high luminance with a lowdriving voltage.

The term singlet compound herein refers to a compound that emits lightsolely through singlet excitation and the term triplet compound hereinrefers to a compound that emits light through triplet excitation.

The luminance of light emitted from an EL element is controlled by thevoltage applied to its EL layer. However, the luminance of emitted lightin relation to the applied voltage varies between luminous materialsused to form the light emitting layer in the EL layer. To elaborate, aluminous material that emits low luminance light requires application ofhigh voltage if a higher luminance is aimed. Unfortunately, applicationof high voltage leads to degradation of the luminous material.Furthermore, if EL elements formed on the same substrate receive thesame voltage but emit light of varying luminance, different voltageshave to be applied in order to make the EL elements to emit light of thesame luminance. This results in another problem of varying EL elementlifetime.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems, and anobject of the present invention is to provide a long-Living EL elementthat can emit Light of desired luminance with a low voltage.

According to the present invention, a plurality of EL elements formed ina pixel portion on the same substrate include EL elements whose ELlayers contain luminous materials emitting low luminance light (singletcompound) and EL elements whose EL layers contain triplet compoundscapable of emitting high luminance Light with a low voltage. By usingthe two types of EL elements in a strategically planned combination, thepresent invention makes it possible to control and equalize theluminance of light emitted from the plural EL elements as well as reducethe power consumption of the EL elements.

FIG. 1A shows a circuit structure of a pixel portion usable in thepresent invention. Reference symbol 101 denotes a gate wiring line, 102a to 102 c, source wiring lines, and 103 a to 103 c, current supplyinglines. These wiring lines define three regions in which a pixel a (104a), a pixel b (104 b), and a pixel c (104 c) are respectively formed.

Denoted by 105 is a switching transistor, which is formed in each of thethree pixels. The structure shown here as an example has two channelformation regions between a source region and a drain region. However,the number of channel formation regions may be more than two or onlyone.

A current controlling transistor is denoted by 106 and is provided ineach pixel. The current controlling transistor has a gate connected toone switching transistor, a source connected to one current supplyingline, and a drain connected to one EL element. Reference symbol 107denotes a condenser, which holds a voltage applied to the gate of thecurrent controlling transistor 106. However, the condenser 107 may beomitted.

The pixel a (104 a), the pixel b (104 b), and the pixel c (104 c) havean EL element a (108 a), an EL element b (108 b), and an EL element c(108 c), respectively.

These EL elements have an element structure shown in FIG. 1B. An ELelement 111 is composed of a cathode 112, an anode 113, and an EL layer114. The EL layer 114 emits light when a voltage is applied to thecathode 112 or the anode 113.

The EL layer 114 consists of a plurality of layers including: a lightemitting layer 115 formed of a luminous material; an electron injectionlayer 116 for improved injection of electrons from the cathode; and anelectron transporting layer 117 for transporting the injected electronsto the light emitting layer 115. The layers 116 and 117 are sandwichedbetween the cathode 112 and the light emitting layer 115.

The EL layer also includes a hole injection layer 118 for improvedinjection of holes from the anode, and a hole transporting layer 119 fortransporting the injected holes to the light emitting layer 115. Thelayers 118 and 119 are sandwiched between the anode 113 and the lightemitting layer 115.

Usually, light is emitted through recombination between the electronsinjected from the cathode 112 and the holes injected from the anode 113taking place in the light emitting layer 115. However, the presentinvention employs a hole transporting layer in order to enhance theluminance of the emitted light. In other words, the invention needs thecathode 112, the anode 113, the light emitting layer 115, and the holetransporting layer but other layers except for the hole transportinglayer are provided only when necessary.

The present invention uses two types of EL elements; one has a tripletcompound in the light emitting layer 115 of the EL layer 114 shown inFIG. 1B and the other has a singlet compound in its light emittinglayer. The two types of EL elements are combined and formed in each ofthe pixels a to c (104 a to 104 c) shown in FIG. 1A, so that theluminance of light emitted from the plural EL elements is equalized anda lopsided degradation in which some EL elements degrade faster thanother EL elements is prevented.

When three color pixel display is intended, for example, if theluminance of light emitted from a luminous material for lighting thepixel a (104 a) in one color is lower than the luminance of light ofother two colors for respectively lighting the pixel b (104 b) and thepixel c (104 c), a triplet compound is used in the light emitting layerof the EL element a (108 a) while singlet compounds are used in thelight emitting layers of the EL elements b and c (108 b and 108 c).

If the luminance of light of two colors for respectively lighting thepixel a (104 a) and the pixel b (104 b) is lower than the luminance oflight of one color for lighting the pixel c (104 c), triplet compoundsare used in the light emitting layers of the EL element a (108 a) andthe EL element b (108 b) while a singlet compound is used in the lightemitting layer of the EL element c (108 c).

If the luminance of emitted light is low in all of three pixels a, b,and c (104 a, 104 b, and 104 c) and higher luminance is wanted to beobtained with a lower voltage, a triplet compound is used in every lightemitting layer of the three EL elements a, b, and c (108 a, 108 b, and108 c).

Materials given as typical triplet compounds are organic compoundsdescribed in the following articles:

(1) T. Tsutsui, C. Adachi, S. Saito, Photochemical Processes inOrganized Molecular Systems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo,1991) p. 437.

(2) M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E.Thompson, S. R. Forrest, Nature 395 (1998), p. 151.

(3) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R.Forrest, Appl. Phys. Lett., 75 (1999) p. 4.

(4) T. Tsutsui, M. J. Yang, M. Yahiro, K Nakamura, T. Watanabe, T.Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38 (12B)(1999) L1502.

Other than the luminous materials described in the articles above, ones20 (specifically, metal complexes or organic compounds) expressed by thefollowing molecular formulae may also be used:

[Chemical Formula 1]

[Chemical Formula 2]

In the above chemical formulae, M represents an element belonging toGroups 8 to 10 in the periodic table and n represents 2 or 3. Platinumor iridium is used in the articles above. Nickel, cobalt, or palladiumis preferable because its physical characteristics are similar to thoseof platinum or iridium. Nickel is particularly preferable as a centralmetal, for it easily forms a complex.

Still another material usable as the triplet compound is a rare earthcomplex which is formed by an ion of a rare earth element, such aseuropium, terbium, or cerium and from a ligand.

The triplet compound has a higher light emission efficiency than thesinglet compound and hence needs lower operation voltage (a voltagerequired to cause an EL element to emit light) in emitting light of thesame luminance.

Furthermore, the present invention improves the mobility of carriers(electrons and holes) injected from an anode by providing a plurality ofhole transporting layers between the anode and a light emitting layer125 as shown in FIGS. 2B and 2C. Although shown in this specification isa case of making only the transporting layer a laminate, the electrontransporting layer may also be a laminate similar to the holetransporting layer. In this case, a layer formed of a compound that canreduce the difference in energy level (LUMO level) is placed between thecathode and the electron transporting layer.

FIG. 2A shows an EL element structure similar to the one shown in FIG.1B. The light emitting layer 125 is placed between a cathode 123 and ananode 124. An electron injection layer 126 and an electron transportinglayer 127 are placed between the cathode 123 and the light emittinglayer 125. A hole injection layer 128 and a hole transporting layer 1(129) are placed between the anode 124 and the light emitting layer 125.

In contrast to this, FIG. 2B shows a laminate structure in which onemore layer namely, a hole transporting layer 2 (130) is added betweenthe hole transporting layer 1 (129) and the hole injection layer 128.

The laminate structure is translated into a band structure of FIG. 2C.Reference symbols used in FIG. 2C are identical with those in FIGS. 2Aand 2B. By the laminate structure formed forming the hole transportinglayer 2 (130) between the hole transporting layer 1 (129) and the holeinjection layer 128 the difference in HOMO level between the holeinjection layer and the hole transporting layer can be reduced. Thisfacilitates movement of holes from the hole injection layer to the holetransporting Layer, and the EL element can have a high luminance with alow voltage as a result.

The case shown here as an example has a laminate structure consisting ofthe hole transporting layer 1 (129) and the hole transporting layer 2(130). However, the laminate structure of the hole transporting layermay have two or more layers formed of different materials if thedifference in HOMO level between the hole injection layer and the holetransporting layer is reduced as mentioned above. Preferably, thelaminate structure has two to five layers.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams illustrating a light emitting device;

FIGS. 2A to 2C are diagrams illustrating a laminate structure of an ELelement;

FIGS. 3A to 3E are diagrams showing a process of manufacturing a lightemitting device;

FIGS. 4A to 4D are diagrams showing a process of manufacturing a lightemitting device;

FIGS. 5A and 5B are diagrams showing a process of manufacturing a lightemitting device;

FIGS. 6A and 6B are diagrams respectively showing a top structure of alight emitting device and a sectional structure thereof;

FIGS. 7A to 7D are diagrams showing laminate structures of EL elements;

FIGS. 8A and 8B are graphs showing element characteristics of ELelements;

FIGS. 9A to 9C are diagrams showing laminate structures of EL elements;

FIGS. 10A and 10B are graphs showing element characteristics of ELelements;

FIGS. 11A to 11C are graphs showing element characteristics of ELelements;

FIG. 12 is a diagram showing a sectional structure of a light emittingdevice;

FIGS. 13A and 13B are diagrams showing the circuit structure of pixelsin a light emitting device;

FIG. 14 is a diagram showing a sectional structure of a light emittingdevice;

FIG. 15 is a diagram showing the circuit structure of pixels in a lightemitting device;

FIG. 16 is a diagram showing a sectional structure of a light emittingdevice;

FIGS. 17A to 17C are diagrams showing a process of manufacturing a lightemitting device;

FIG. 18 is a diagram showing the circuit structure of pixels in a lightemitting device;

FIGS. 19A and 19B are diagrams showing the structure of a light emittingdevice with external driving circuit;

FIGS. 20A and 20B are diagrams showing the structure of a light emittingdevice with external controller;

FIGS. 21A to 21F are diagrams showing specific examples of an electricmachine;

FIGS. 22A to 22F are diagrams showing specific examples of an electricmachine;

FIGS. 23A and 23B are diagrams respectively showing a top structure of alight emitting device and a sectional structure thereof;

FIG. 24 is a diagram showing the circuit structure of pixels in a lightemitting device; and

FIG. 25 is a diagram showing element characteristics of an EL element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Modes for carrying out the present invention will be described in detailthrough the following embodiments.

Embodiment 1

In this embodiment, a description will be given of a method ofmanufacturing a pixel portion and a driving circuit provided at itsperiphery on the same insulator. However, for simplification of thedescription, with respect to the driving circuit, a CMOS circuit inwhich an n-channel transistor and a p-channel transistor are combinedwill be shown.

First, as shown in FIG. 3A, a glass substrate 201 is prepared as ainsulator. In this embodiment, not-shown protection films (carbon films,specifically diamond-like carbon films) are provided on both surfaces(the front surface and the rear surface) of the glass substrate 201. Aslong as it is transparent to visible light, a material other than glass(for example, plastic) may be used.

Next, an base film 202 having a thickness of 300 nm is formed on theglass substrate 201. In this embodiment, as the base film 202, siliconoxynitride films are laminated and are used. At this time, it isappropriate that the concentration of nitrogen of a layer adjacent tothe glass substrate 201 is made 10 to 25 wt %, and nitrogen is made tobe contained at the concentration rather higher than that of anotherlayer.

Next, an amorphous silicon film (not shown) having a thickness of 50 nmis formed on the base film 202 by a sputtering method. Note that, it isnot necessary to limit the film to the amorphous silicon film, but anysemiconductor films (including a microcrystalline semiconductor film)containing amorphous structure may be used. As the amorphoussemiconductor film, an amorphous silicon film or an amorphous silicongermanium film (a silicon film containing germanium at a concentrationof 1×10¹⁸ to 1×10²¹ atoms/cm³) may be used. The film thickness may be 20to 100 nm.

Then, crystallization of the amorphous silicon film is performed byusing a well-known laser crystallizing method, and a crystalline siliconfilm 203 is formed. In this embodiment, although a solid laser(specifically, second harmonic of Nd:YAG laser) is used, an excimerlaser may also be used. As the crystallizing method, a furnace annealingmethod may be used.

Next, as shown in FIG. 3B, the crystalline silicon film 203 is etched bya first photolithography step to form island-like crystalline siliconfilms 204 to 207. These are crystalline silicon films which subsequentlybecome the active layers of transistors.

Note that, in this embodiment, although the crystalline silicon filmsare used as the active layers of the transistors, an amorphous siliconfilm can also be used as the active layer.

Here, in this embodiment, a protection film (not shown) made of asilicon oxide film and having a thickness of 130 nm is formed on theisland-like crystalline silicon films 204 to 207 by a sputtering method,and an impurity element (hereinafter referred to as a p-type impurityelement) to make a semiconductor a p-type semiconductor is added to theisland-like crystalline silicon films 204 to 207. As the p-type impurityelement, an element (typically, boron or gallium) belonging to group 13of the periodic table can be used. Note that, this protection film isprovided to prevent the crystalline silicon film from directly beingexposed to plasma when the impurity is added, and to enable fineconcentration control.

The concentration of the p-type impurity element added at this time maybe made 1×10¹⁵ to 5×10¹⁷ atoms/cm³ (typically, 1×10¹⁶ to 1×10¹⁷atoms/cm³). The p-type impurity element added at this concentration isused to adjust the threshold voltage of the n-channel transistor.

Next, the surfaces of the island-like crystalline silicon films 204 to207 are washed. First, the surface is washed by using pure watercontaining ozone. At that time, since a thin oxide film is formed on thesurface, the thin oxide film is removed by using a hydrofluoric acidsolution diluted to 1%. By this treatment, contaminants adhered to thesurfaces of the island-like crystalline silicon films 204 to 207 can beremoved. At this time, it is preferable that the concentration of ozoneis 6 mg/L or more. The series of treatments are carried out withoutopening to the air.

Then, a gate insulating film 208 is formed to cover the island-likecrystalline silicon films 204 to 207. As the gate insulating film 208,an insulating film having a thickness of 10 to 150 nm, preferably 50 to100 nm and containing silicon may be used. This may have a single-layerstructure or a laminate structure. In this embodiment a siliconoxynitride film having a thickness of 80 nm is used.

In this embodiment, the steps from the surface washing of theisland-like crystalline silicon films 204 to 207 to the formation of thegate insulating film 208 are carried out without opening to the air, sothat contaminants and interface levels on the interface between thesemiconductor film and the gate insulating film are lowered. In thiscase, a device of a multi-chamber system (or an inline system) includingat least a washing chamber and a sputtering chamber may be used.

Next, a tantalum nitride film having a thickness of 30 nm is formed as afirst conductive film 209, and further, a tungsten film having athickness of 370 nm is formed as a second conductive film 210. Inaddition, a combination of a tungsten film as the first conductive filmand an aluminum alloy film as the second conductive film, or acombination of a titanium film as the first conductive film and atungsten film as the second conductive film may be used.

These metal films may be formed by a sputtering method. When an inertgas such as Xe or Ne is added as a sputtering gas, film peeling due tostress can be prevented. When the purity of a tungsten target is made99.9999%, a low resistance tungsten film having a resistivity of 20 mΩcmor less can be formed.

Besides, the steps from the surface washing of the semiconductors 204 to207 to the formation of the second conductive film 210 can also becarried out without opening to the air. In this case, a device of amulti-chamber system (or an inline system) including at least a washingchamber, a sputtering chamber for forming an insulating film, and asputtering chamber for forming a conductive film may be used.

Next, the resist 211a to 211e is formed and the second conductive film210 is etched. As an etching condition, it is preferable to perform adry etching using ICP (Inductively Coupled Plasma). As an etching gas, amixture gas of a carbon tetrafluoride (CF₄) gas, a chlorine (Cl₂) gasand an oxygen (O₂) gas is used.

As a typical etching condition, a gas pressure is made 1 Pa, and in thisstate, RF electric power (13.56 MHz) of 500 W is applied to a coil typeelectrode to produce plasma. Besides, RF electric power (13.56 MHz) of150 W is applied as a self bias voltage to a stage on which thesubstrate is put, so that a negative self bias is applied to thesubstrate. At this time, it is appropriate that the amount of the flowof the respective gases is made such that the carbon tetrafluoride gashas a flow of 2.5×10⁻⁵ m³/min, the chlorine gas has a flow of 2.5×10⁻⁵m³/min, and the oxygen gas has a flow of 1.0×10⁻⁵ m³/min. (FIG. 3C)

By this, the second conductive film (tungsten film) 210 is selectivelyetched, and electrodes 212 to 216 made of the second conductive film areformed. The reason why the second conductive film 210 is selectivelyetched is that the progress of etching of the first conductive film(tantalum nitride film) becomes extremely slow by addition of oxygen tothe etching gas.

Note that, here, there is a reason why the first conductive film 209 ismade to remain. Although the first conductive film can also be etched atthis time, if the first conductive film is etched, the gate insulatingfilm 208 is also etched in the same step and the film thickness isdecreased. At this time, if the thickness of the gate insulating film208 is 100 nm or more, there is no problem. However, if the thickness isless than that, a part of the gate insulating film 208 is removed in asubsequent step and the semiconductor film thereunder is exposed, andthere is a possibility that the semiconductor film which becomes asource region or a drain region of a transistor is also removed.

However, the foregoing problem can be solved by leaving the firstconductive film 209 as in this embodiment.

Next, an n-type impurity element (in this embodiment, phosphorus) isadded in a self-aligning manner by using the resists 211 a to 211 e andthe electrodes 212 to 216. At this time, phosphorus passes through thefirst conductive film 209 is added. Impurity regions 217 to 225 formedin this way contain the n-type impurity element at a concentration of1×10²⁰ to 1×10²¹ atoms/cm³ (typically 2×10²⁰ to 5×10²¹ atoms/cm³).

Next, the first conductive film 209 is etched using a resists 211 a to211 e as masks. This etching is performed by a dry etching method usingthe ICP, and a mixture gas of a carbon tetrafluoride (CF₄) gas and achlorine (Cl₂) gas is used as an etching gas. A typical etchingcondition is such that a gas pressure is made 1 Pa, and RF electricpower (13.56 MHz) of 500 W is applied to a coil type electrode toproduce plasma in this state. Besides, RF electric power (13.56 MHz) of20 W is applied as a self bias voltage to the stage on which thesubstrate is put, so that a negative self bias is applied to thesubstrate. At this time, it is appropriate that the flow of therespective gases is made such that the carbon tetrafluoride gas has aflow of 3.0×10⁻⁵ m³/min, and the chlorine gas has a flow of 3.0×10⁻⁵m³/min. Thus, the electrodes 226 to 230 from the first conductive filmare formed. (FIG. 3D)

Next, as shown in FIG. 3E, the electrodes 212 to 216 from the secondconductive film is etched selectively using the resists 211 a to 211 e.This etching is performed by a dry etching method using the ICP, and amixture gas of a carbon tetrafluoride (CF₄) gas, a chlorine (Cl₂) gasand an oxygen (O₂) gas is used as an etching gas. A typical etchingcondition is such that a gas pressure is made 1 Pa, and in this state,RF electric power (13.56 MHz) of 500 W is applied to a coil typeelectrode to produce plasma. Besides, RF electric power (13.56 MHz) of20 W is applied as a self bias voltage to the stage on which thesubstrate is put, so that a negative self bias is applied to thesubstrate. At this time, it is appropriate that the amount of the flowof the respective gases is made such that the carbon tetrafluoride gashas a flow of 2.5×10⁻⁵ m³/min, the chlorine gas has a flow of 2.5×10⁻⁵m³/min, and the oxygen gas has a flow of 1.0×10⁻⁵ m³/min. The etchingrate of the tantalum nitride film is suppressed by the existence ofoxygen. Thus, the second gate electrodes 231 to 235 are formed.

Next, an n-type impurity element (in this embodiment, phosphorus) isadded. In this step, the second gate electrodes 231 to 235 function asmasks, and phosphorus passes through part of the electrodes 226 to 230made of the first conductive film and is added, and n-type impurityregions 236 to 245 containing phosphorus at a concentration of 2×10¹⁶ to5×10¹⁹ atoms/cm³ (typically, 5×10¹⁷ to 5×10¹⁸ atoms/cm³) are formed.

Besides, as an addition condition here, an acceleration voltage is setquite high as 70 to 120 kV (in this embodiment, 90 kV) so thatphosphorus passes through the first conductive film and the gateinsulating film and reaches the island-like crystalline silicon films.

Next, as shown in FIG. 4A, the electrodes 226 to 230 made of the firstconductive film are etched to form first gate electrodes 246 to 250.This etching is performed by a dry etching method using the ICP or a dryetching method with an RIE (Reactive Ion Etching) mode, and a mixturegas of a carbon tetrafluoride (CF₄) gas and a chlorine (Cl₂) gas is usedas an etching gas. A typical etching condition is such that a gaspressure is made 1 Pa, and RF electric power (13.56 MHz) of 500 W isapplied to a coil type electrode to produce plasma in this state.Besides, RF electric power (13.56 MHz) of 20 W is applied as a self biasvoltage to the stage on which the substrate is put, so that a negativeself bias is applied to the substrate. At this time, it is appropriatethat the amount of the flow of the respective gases is made such thatthe carbon tetrafluoride gas has a flow of 2.5×10⁻⁵ m³/min, the chlorinegas has a flow of 2.5×10⁻⁵ m³/min, and the oxygen gas has a flow of1.0×10⁻⁵ m³/min.

At this time, the first gate electrodes 246 to 250 are etched so thatthey partially overlap the n-type impurity regions 236 to 245 throughthe gate insulating film 208. For example, the n-type impurity region236 is divided into a region 236 a not overlapping the first gateelectrode 246 and a region 236 b overlapping there through the gateinsulating film 208. The n-type impurity region 237 is divided into aregion 237 a not overlapping the first gate electrode 246 and a region237 b overlapping there through the gate insulating film 208.

Next, resists 251 a and 251 b are formed, and an impurity element(hereinafter referred to as a p-type impurity element) to make asemiconductor a p-type semiconductor is added. As the p-type impurityelement, an element (typically, boron) belonging to group 13 of theperiodic table may be added. Here, an acceleration voltage is set sothat boron passes through the first gate electrodes 247 and 250 and thegate insulating film 208, and reaches the semiconductor film. In thisway, p-type impurity regions 252 to 255 are formed (FIG. 4B).

Next, as shown in FIG. 4C, as a first inorganic insulating film 56, asilicon nitride film or silicon oxynitride film having a thickness of 30to 100 nm is formed. Thereafter, the added n-type impurity element andp-type impurity element are activated. As an activation means, a furnaceannealing, a laser annealing, a lamp annealing, or a combination ofthose can be used.

Next, as shown in FIG. 4D, a second inorganic insulating film 257 madeof a silicon nitride film or a silicon oxynitride film is formed to athickness of 50 to 200 nm. After the second inorganic insulating film257 is formed, a heat treatment in the temperature range of 350 to 450°C. is carried out. Note that, it is effective to carry out a plasmatreatment using a hydrogen (H₂) gas or an ammonia (NH₃) gas before thesecond inorganic insulating film 257 is formed.

Next, as an organic insulating film 258, a resin film transparent tovisible light is formed to a thickness of 1 to 2 μm. As the resin film,a polyimide film, a polyamide film, an acryl resin film, or a BCB(benzocyclobutene) film may be used. Besides, a photosensitive resinfilm can also be used.

Note that, in this embodiment, the laminate film of the first inorganicinsulating film 256, the second inorganic insulating film 257, and theorganic insulating film 258 is generically called an interlayerinsulating film.

Next, as shown in FIG. 5A, a pixel electrode (anode) 259 made of anoxide conductive film which has a large work function and is transparentto visible light is formed to a thickness of 80 to 120 nm on the organicinsulating film 258. In this embodiment, an oxide conductive film inwhich gallium oxide is added to zinc oxide is formed. Besides, asanother oxide conductive film, it is also possible to use an oxideconductive film made of indium oxide, zinc oxide, tin oxide, or acompound of combination of those as other oxide conductive film.

Note that, after the oxide conductive film is formed, althoughpatterning is carried out to form the pixel electrode 259, a flatteningtreatment of the surface of the oxide conductive film can also becarried out before the patterning. The flattening treatment may be aplasma treatment or a CMP (Chemical Mechanical Polishing) treatment.

Next, contact holes are formed in the interlayer insulating film, andwiring lines 260 to 266 are formed. At this time, the wiring line 266 isformed to be connected with the pixel electrode 259. In this embodiment,this wiring line is made as the laminate film of three-layer structurein which a titanium film having a thickness of 150 nm, an aluminum filmcontaining titanium and having a thickness of 300 nm and a titanium filmhaving a thickness of 100 nm are continuously formed from the lowerlayer side by a sputtering method.

At this time, the wiring lines 260 and 262 function as source wiringlines of a CMOS circuit, and the wiring line 261 functions as a drainwiring line. The wiring line 263 is a source wiring line of a switchingtransistor, and the wiring line 264 is a drain wiring line of theswitching transistor. The wiring line 265 is a source wiring line(equivalent to a current supply line) of a current controllingtransistor, and the wiring line 266 is a drain wiring line of thecurrent controlling transistor and is connected with the pixel electrode259.

Next, as shown in FIG. 5B, a bank 267 is formed. The bank 267 may beformed by patterning an insulating film having a thickness of 100 to 400nm and containing silicon or an organic resin film. This bank 267 isformed to fill a portion between pixels (between pixel electrodes).Besides, it also has an object to prevent a subsequently formed organicEL film such as a light emitting layer from being brought into directcontact with the end portion of the pixel electrode 259.

Incidentally, since the bank 267 is an insulating film, attention mustbe paid to electrostatic damage of a device at the time of filmformation. When carbon particles or metal particles are added into theinsulating film, which becomes a material of the bank 267, to lower itsresistivity, the generation of static electricity at the time of filmformation can be suppressed. In that case, it is appropriate that theamount of addition of carbon particles or metal particles is adjusted sothat the resistivity of the insulating film, which becomes a material ofthe bank 267, becomes 1×10⁶ to 1×10¹² Ωm (preferably, 1×10⁸ to 1×10¹⁰Ωm).

When the carbon particles or the metal particles are added to the bank267, optical absorption is raised and transmissivity is lowered. Thatis, since light from the outside of the light emitting device isabsorbed, it is possible to avoid such a disadvantage that an outsidescene is reflected in the cathode surface of the EL element.

Next, an EL layer 268 is formed by evaporation. In this embodiment, alaminate of a hole injection layer and a light emitting, layer is calledan EL layer. An EL layer could be a laminate obtained by combining alight emitting layer with a hole injection layer, a hole transportinglayer, a hole blocking layer, an electron transporting layer, and anelectron injection layer. As long as the laminate includes a lightemitting layer and a hole transporting layer, it fulfills the definitionof the EL layer in this specification.

Described here is a method of forming a light emitting layer that emitsgreen light from a triplet compound in the light emitting layer as theEL layer.

A copper phthalocyanine (CuPc) film with a thickness of 20 nm is formedfirst as a hole injection layer in this embodiment. Then, as a holetransporting layer, MTDATA that is an aromatic amine called star burstamine is deposited to a thickness of 20 nm and α-NPD that is also anaromatic amine-based compound is deposited to a thickness of 10 nm. Thusthe hole transporting layer described in this embodiment has a two-layerstructure of MTDATA and α-NPD.

Materials for forming the hole transporting layer are roughly dividedinto hole transporting low molecular weight compounds and holetransporting high molecular weight compounds. One or more compounds areselected from each of the two types of compounds to form a laminate holetransporting layer. Specifically TPAC, PDA, TPD, and like othercompounds can be used as the hole transporting low molecular weightcompounds whereas various high polymers having polyvinyl carbazole (PVK)or TPD as their principal chains or side chains can be used as the holetransporting high molecular weight compounds.

The hole transporting layer thus can have layers formed of differentmaterials. However, the total thickness of the hole transporting layeris preferably about 20 to 100 nm. When the layers that constitute thehole transporting layer are increased in number the thickness of theindividual layers has to be reduced. Therefore two to four layers arepreferable.

Then a light emitting layer is formed from CBP and Ir(ppy)₃ byco-evaporation to a thickness of 20 nm. After the light emitting layeris formed, a hole blocking layer is formed from BCP to a thickness of 10nm and an electron transporting layer is formed from analuminoquinolilate complex (Alq₃) to a thickness of 40 nm.

The case described here is of forming an EL layer that emits greenlight. Examples of other usable luminous materials emitting green lightinclude an aluminoquinolilate complex (Alq₃), which is given in theabove as the material of the electron transporting layer, and aberyllium benzoquinolilate complex (BeBq). Also included in the examplesis an aluminoquinolilate complex (Alq₃) doped with coumarin 6 orquinacridon.

When an EL layer emitting red light is to be formed, examples of theusable luminous material include an Eu complex (Eu(DCM)₃ (Phen)) and analuminoquinolilate complex (Alq₃) that is doped with DCM-1.

When an EL layer emitting blue light is to be formed, examples of theusable luminous material include DPVBi that is a distyril derivative, azinc complex having an azomethine compound as a ligand, and DPVBi dopedwith perylene.

In carrying out the present invention, the luminous materials given inthe above can be used to form EL layers respectively emitting red lightgreen light, and blue light, for example. A singlet compound and atriplet compound can be used in any combination as luminous materials inaccordance with the need. Materials introduced in ‘Summary of theInvention’ may also be used as a triplet compound.

The EL layers respectively emitting red light, green light, and bluelight formed here are merely an embodiment. Color of emitted light isnot limited thereto and combinations of other colors can be chosen.

After the EL layer 268 is formed, a cathode 269 is formed to a thicknessof 300 nm from a conductive film having a small work function. Aconductive film containing an element belonging to Group 1 or 2 in thelong-period periodic table and a transition element belonging to Groups3 through 11 can be used as a conductive film having a small workfunction. This embodiment uses a conductive film formed of ytterbium(Yb). A conductive film formed of a compound of lithium and aluminum mayalso be used. Thus completed is an EL element 270 including the pixelelectrode (anode) 259, the EL layer 268, and the cathode 269.

After the cathode 269 is formed, it is effective to form a passivationfilm 271 so as to completely cover the EL element 270. The passivationfilm 271 is a single layer of insulating film or a laminate of acombination of insulating films. Examples of the insulating film includea carbon film, a silicon nitride film, and a silicon oxynitride film.

A preferred passivation film is one that can cover a wide area, and acarbon film especially a DLC (diamond-like carbon) film, is effective. ADLC film can be formed at a temperature range of from room temperatureto 100° C., and it is easily be formed above the EL layer 268 that has alow heat resistance. In addition, a DLC film is high in oxygen blockingeffect and can prevent oxidization of the EL layer 268. Therefore,oxidization of the EL layer 268 during the subsequent sealing step canbe avoided.

A seal (not shown in the drawing) is provided on the substrate 201 (oron the base film 202) so as to surround at least the pixel portion,thereby bonding a covering member 272. The seal may be a UV-curableresin which allows less amount of gas to free and through which moistureand oxygen are hardly transmitted. A gap 273 is filled with inert gas(nitrogen gas or rare gas) or a resin (UV-curable resin or epoxy resin).

It is effective to place a substance having a hygroscopic effect or asubstance having an antioxidizing effect in the gap 273. The coveringmember 272 may be a glass substrate, a metal substrate (preferably astainless steel substrate), a ceramic substrate, or a plastic substrate(including a plastic film). When a plastic substrate is used, it ispreferable to form carbon films (preferably diamond-like carbon films)on the front and back surfaces of the substrate to prevent transmissionof oxygen or moisture.

A light emitting device structured as shown in FIG. 5B is thuscompleted. It is effective to use a film formation apparatus ofmulti-chamber type or inline type to process steps subsequent toformation of the bank 267 through formation of the passivation film 271in succession without exposing the device to the air. The successiveprocessing may be further extended to the step of bonding the coveringmember 272 while avoiding exposure to the air.

Thus formed on the substrate 201 are an n-channel transistor 601, ap-channel transistor 602, a switching transistor (a transistorfunctioning as a switching element for transferring a video data signalto a pixel) 603, and a current controlling transistor (a transistorfunctioning as a current controlling element for controlling a currentflowing into an EL element) 604.

The driving circuit here includes as a basic circuit a CMOS circuit thatcombines the n-channel transistor 601 and the p-channel transistor 602complementarily. The pixel portion is composed of a plurality of pixelseach including the switching transistor 603 and the current controllingtransistor 604.

Up to this point, the manufacture process has needed thephotolithography processing seven times, which is less than in a generalactive matrix light emitting device. In other words, the process ofmanufacturing transistors is greatly simplified to improve the yield andreduce the manufacture cost.

Moreover, as explained referring to FIG. 4, by preparing an impurityregion that overlaps a first gate electrode with a gate insulating filminterposed therebetween, the n-channel transistor can be formed which isstrong against degradation due to hot carrier injection. Therefore, alight emitting device of high reliability can be provided.

The light emitting device of this embodiment which has been finished upthrough the sealing (or enclosing) step for protecting the EL element isfurther described with reference to FIGS. 6A and 6B. The symbols used inFIGS. 3A to 5B are mentioned when necessary.

FIG. 6A is a top view showing the device that has been finished upthrough sealing the EL element, and FIG. 6B is a sectional view takenalong the line A-A′ in FIG. 6A. An area surrounded by a dotted line anddenoted by 501 is a pixel portion, and 502 and 503 represent a sourceside driving circuit and a gate side driving circuit, respectively.Denoted by 504, 505, and 506 are a covering member, a first seal, and asecond seal respectively.

Reference symbol 507 denotes a wiring line for transferring signals tobe inputted to the source side driving circuit 502 and the gate sidedriving circuit 503. The wiring line 508 receives video signals andclock signals from an FPC (flexible printed circuit) 508 that is anexternal input terminal. Although the FPC alone is shown in FIG. 6A, aprinted wiring board (PWB) may be attached to the FPC.

The sectional structure is described next referring to FIG. 6B. Thepixel portion 501 and the source side driving circuit 502 are formedover the substrate 201. The pixel portion 501 is composed of a pluralityof pixels each including the current controlling transistor 604 and thepixel electrode 259 electrically connected to the drain of thetransistor 604. The source side driving circuit 502 is composed of aCMOS circuit that combines the n-channel transistor 601 and thep-channel transistor 602 (see FIG. 5B). A polarizing plate (typically acircular polarizing plate) may be bonded to the substrate 201.

The pixel electrode 259 functions as the anode of the EL element. Thebank 267 is formed on each end of the pixel electrode 259. The EL layer268 is formed on the pixel electrode 259 and the cathode 269 of the ELelement is formed on the EL layer. The cathode 269 also functions as awiring line common to all the pixels, and is electrically connected tothe FPC 508 through the connection wiring line 507. All the elementsincluded in the pixel portion 501 and the source side driving circuit502 are covered with the passivation film 271.

The covering member 504 is bonded by the first seal 505. A spacer may beprovided to secure the distance between the covering member 504 and theEL element. The gap 273 is provided inside the first seal 505. The firstseal 505 is desirably a material that does not transmit moisture andoxygen. It is effective to place a substance having a hygroscopic effector a substance having an antioxidizing effect in the gap 273.

On the front and back surfaces of the covering member 504, carbon films(specifically diamond-like carbon films) 509 a and 509 b each having athickness of 2 to 30 nm are formed as protective films. The carbon filmsmechanically protect the surfaces of the covering member 504 as well asprevent permeance of oxygen and moisture.

After the covering member 504 is bonded the second seal 506 is placed soas to cover the exposed surfaces of the first seal 505. The samematerial may be used for the second seal 506 and the first seal 505.

By enclosing the EL element with the structure as above, the EL elementcan be shut off from the surroundings completely and external substancesthat accelerate degradation by oxidization of the EL layer, such asmoisture and oxygen, can be prevented from entering the EL element.Accordingly, a light emitting device of high reliability can beobtained.

A light emitting device in which a pixel portion and a driving circuitare on the same substrate and an FPC is attached to the substrate asshown in FIGS. 6A and 6B is specially called a light emitting devicewith built-in driving circuit in this specification.

The light emitting device manufactured in accordance with thisembodiment can operate on both digital signals and analog signals.

Embodiment 2

This embodiment shows characteristics of EL elements having different ELlayers that can be used in carrying out the present invention.Structures of the EL layers formed in this embodiment are shown in FIGS.7A to 7D.

FIG. 7A shows the structure of an EL element a. First, a holetransporting layer is formed from α-NPD by evaporation to a thickness of40 nm on an anode that is formed of a compound of indium oxide and tinoxide. On the hole transporting layer, a light emitting layer is formedfrom luminous materials consisting of Ir(ppy)₃ and CBP (tripletcompounds) by co-evaporation to a thickness of 20 nm. On the lightemitting layer, a BCP layer with a thickness of 10 nm and a ALq₃ layerwith a thickness of 40 nm are formed by evaporation as an electrontransporting layer. Then a cathode is formed from Yb to a thickness of400 nm to complete the EL element a. Light emission from the EL elementa utilizes triplet excitation energy by the triplet compounds.

FIG. 7B shows the structure of an EL element b. First, a hole injectionlayer is formed from copper phthalocyanine by evaporation to a thicknessof 20 nm on an anode that is formed of a compound of indium oxide andtin oxide. A hole transporting layer is formed thereon by depositingMTDATA to a thickness of 20 nm and then depositing α-NPD to a thicknessof 10 nm by evaporation. On the hole transporting Layer, a lightemitting layer is formed from a luminous material consisting of Alq₃(singlet compound) by evaporation to a thickness of 50 nm. Then acathode is formed from Yb to a thickness of 400 nm to complete the ELelement b by evaporation. Light emission from the EL element b utilizessinglet excitation energy by the singlet compound.

FIG. 7C shows the structure of an EL element c. First, a holetransporting layer is formed from α-NPD by evaporation to a thickness of50 nm on an anode that is formed of a compound of indium oxide and tinoxide. On the hole transporting layer, a light emitting layer is formedfrom a luminous material consisting of Alq₃ (singlet compound) byevaporation to a thickness of 50 nm. Then a cathode is formed from Yb toa thickness of 400 nm to complete the EL element c. Light emission fromthe EL element c utilizes singlet excitation energy by the singletcompound. The EL layer of the EL element c has no other layers than thelight emitting layer and the hole transporting layer.

FIG. 7D shows the structure of an EL element d. First, a holetransporting layer is formed from PEDOT that is a polythiophenederivative by spin coating to a thickness of 30 nm on an anode that isformed of a compound of indium oxide and tin oxide.Polyparaphenylenevinylene (hereinafter referred to as PPV) is then usedas a luminous material to form a film with a thickness of 80 nm by spincoating on the hole transporting layer. Then a cathode is formed from Ybto a thickness of 400 nm to complete the EL element d by evaporation.Light emission from the EL element d utilizes singlet excitation energyby the singlet compound. The EL element d is different from the other ELelements a to c in that a high molecular weight material is used for thelight emitting layer.

The EL elements illustrated in FIGS. 7A to 7D have been estimated fortheir electrical characteristics. Results are shown in FIGS. 8A and 8B.FIG. 8A shows the luminance characteristic in relation to the currentdensity rough observation, there is a difference in characteristic inrelation to the current density between the EL element that uses tripletcompounds and the EL elements that use singlet compounds. To elaborate,when the current density is 60 mA/cm², the EL element a that usestriplet compounds provides a luminance of about 6000 cd/m² whereas theEL elements b, c and d that use singlet compounds each provide aluminance of about 2000 cd/m², namely, one third of the luminance of theEL element a.

FIG. 8B shows results of measuring the external quantum efficiency inrelation to the current density. Similar to the case of the luminancecharacteristic, the EL element a that uses triplet compounds hasexhibited a far better external quantum efficiency. The difference inexternal quantum efficiency between the EL element a and the EL elementsb to d is seven times at the maximum.

As shown in the results in FIGS. 8A and 8B, employing a triplet compoundin an EL element improves light emission efficiency.

In order to further improve light emission provided by the EL element aof FIG. 7A which uses triplet compounds, another layer is added to theelement.

FIG. 9A shows the same EL element a as the one shown in FIG. 7A. In FIG.9B, copper phthalocyanine is deposited by evaporation to a thickness of20 nm on the anode of the EL element a. Electric characteristics of thisEL element is shown in FIGS. 10A and 10B. As shown in FIG. 10A,providing the copper phthalocyanine layer on the anode does not changethe luminance of the EL element itself much but the time during whichthe luminance is maintained is prolonged.

FIG. 10B shows that the amount of current flowing in an early stage ischanged by addition of one more layer but eventually reaches the samevalue. Therefore, it is clear from FIGS. 10A and 10B that the durabilityof the EL element when the same amount of current is flown is improved.Although copper phthalocyanine is usually known as a hole injectionlayer material that improves injection of holes from the anode, it isused here as a material that can improve the durability of the ELelement. The results are obtained by measuring a change with time of theluminance of the EL element and a change with time of the amount ofcurrent flowing through the EL element when the EL element iscontinuously lit using a low voltage of 6.5 V. Instead of copperphthalocyanine shown in this embodiment, a polythiophene-based material,for example, PEDOT (poly(3,4-ethylene dioxythiophene)), may be used.

Then an EL element shown in FIG. 9C is fabricated. This EL element hasinstead of the α-NPD hole transporting layer (40 nm) of FIG. 9B anMTDATA layer with a thickness of 20 nm and an α-NPD layer with athickness of 10 nm which are formed by evaporation. In short, one morelayer is formed between the copper phthalocyanine layer and the holetransporting layer, thereby reducing the energy difference in HOMO levelbetween the two layers. The element in FIG. 9C is referred to as ELelement a′ in this specification.

Electrical characteristics of the EL element of FIG. 9C is shown inFIGS. 11A to 11C. FIG. 11A shows results of measuring the luminance ofemitted light in relation to the current density. The measurement ismade on the EL element a shown in FIG. 9A and the EL element a′ obtainedby adding, to the EL element a, a hole injection layer formed of copperphthalocyanine and a hole transporting layer formed of MTDATA. From FIG.11A, it can be seen that the addition of the copper phthalocyanine layerand the MTDATA layer does not influence the luminance of light emittedfrom the EL element.

FIG. 11B shows results of measuring the luminance of emitted light whena voltage is applied to the EL elements. An improvement is observed inluminance which is brought by the addition of the copper phthalocyaninelayer and the MTDATA layer. The fact that a higher luminance is obtainedfrom application of the same voltage means a lower voltage is needed toobtain the same level of luminance.

FIG. 11C shows results of measuring the amount of current when a voltageis applied to the EL elements. When the same voltage is applied, theamount of current flowing is larger in the EL element a′ than in the ELelement a.

The results above state that the voltage required to drive an EL elementis reduced by adding to the EL element a the copper phthalocyanine layerand the MTDATA layer (EL element a′).

The EL element a′ has been measured also for its response speed.

In the measurement, DC (direct current) is applied by an arbitrary powersupply. A period during which the voltage is applied is ‘ON’ (selectedperiod) whereas a period during which 0 V is applied is ‘OFF’(not-selected period), and ON and OFF take turns. Each period lasts 250μs.

To be specific, estimation is made by using an oscilloscope to readoutputs of a photomultiplier set in a microscope. In this measurement, aswitching from OFF to ON is defined as rise and a switching from ON toOFF as drop. The rise response time is a time required for the emittedlight to reach 90% luminance of full luminance in an optical responsethat follows switching of the power supply voltage from OFF to ON. Onthe other hand, the drop response time is a time required for theemitted light to decrease in luminance by 10% of the previous fullluminance in an optical response that follows switching of the powersupply voltage from ON to OFF.

The measurement is graphically shown in FIG. 25. In FIG. 25, an arrow aindicates the output (voltage) of the power supply and an arrow bindicates the optical response to the output. The photomultiplier usedis of minus output type, and a negative electric potential is thereforeoutputted when a switching is made from OFF (0 V) to ON (6 V in theexample shown here).

An arrow c in FIG. 25 indicates the point at which the luminance reaches90%. The rise response time at this point is 28 μs. In this embodiment,when the output of the power supply is 6 V, although there are slightfluctuations between the EL elements, the rise response time and thedrop response time are both 1 to 100 μs, preferably 1 to 50 μs. Furthermeasurement is made by changing the voltage during ON so that estimationis made for every voltage between 6 V and 10 V. Results thereof (therise response time and the drop response time) are shown in Table 1.

[Table 1]

Table 1 shows that the response speed in this voltage range is very highand that the element therefore has no problem also when driven by normaldigital driving.

Embodiment 3

FIG. 12 shows a sectional structure of a pixel portion in an activematrix light emitting device of this embodiment. In FIG. 12, referencesymbol 10 denotes an insulator, 11, the current controlling transistor(TFT) 604 of FIG. 5B. 12, a pixel electrode (anode), 13, a bank, and 14,a known hole injection layer. Reference symbols 15, 16, and 17 representa light emitting layer that emits red light, a light emitting layer thatemits green light, and a light emitting layer that emits blue light,respectively. Denoted by 18 is a known electron transporting layer, and19, a cathode.

In this embodiment, triplet compounds are used for the red lightemitting layer 15 and the blue light emitting layer 17 whereas a singletcompound is used for the green light emitting layer 16. In other words,an EL element that uses a singlet compound is an EL element that emitsgreen light while EL elements that use triplet compounds are an ELelement that emits red light and an EL element that emits blue light.

When a low molecular weight organic compound is used for a lightemitting layer, a red light emitting layer and a blue light emittinglayer have a lifetime shorter than that of a green light emitting layerunder the present circumstances. This is because the red light emittinglayer and the blue light emitting layer are inferior in light emissionefficiency to the green light emitting layer and hence require higheroperation voltage in order to emit light of the same luminance as thegreen light, to thereby accelerate their degradation that much.

However, the red light emitting layer 15 and the blue light emittinglayer 17 in this embodiment use triplet compounds that are high in lightemission efficiency and hence it is possible to obtain the sameoperation voltage as the green light emitting layer 16 in emitting lightof the same level of luminance as the layer 16. Accordingly, the redlight emitting layer 15 and the blue light emitting layer 17 degrade notso much faster than the green light emitting layer 16, and an image canbe displayed in color while avoiding color displacement and like otherproblems. The lowered operation voltage is also preferable in terms ofthe margin for the withstand voltage of the transistor because themargin can be set low.

Although the case shown in this embodiment is of using triplet compoundsfor the red light emitting layer 15 and the blue light emitting layer17, the green light emitting layer 16 may also be formed of a tripletcompound.

Next, the circuit structure of the pixel portion according to thisembodiment is shown in FIGS. 13A and 13B. Shown here are a pixel (pixel(RED)) 20 a having an EL element that emits red light, a pixel (pixel(GREEN)) 20 b having an EL element that emits green light, and a pixel(pixel (BLUE)) 20 c having an EL element that emits blue light. Thethree pixels have the same circuit structure.

In FIG. 13A, reference symbol 21 denotes a gate wiring line, 22 a to 22c, source wiring lines (data wiring lines), and 23 a to 23 c, currentsupplying lines. The current supplying lines 23 are wiring lines thatdetermine the operation voltage of the EL elements, and apply the samevoltage to the red light emitting pixel 20 a, the green light emittingpixel 20 b, and the blue light emitting pixel 20 c. Accordingly, thewiring lines may be designed to have the same width (thickness).

Denoted by 24 a to 24 c are switching transistors, which are n-channeltransistors in this embodiment. Although shown here as an example is astructure in which two channel formation regions are placed between asource region and a drain region, the number of channel formationregions may be more than two or only one.

Denoted by 25 a to 25 c are current controlling transistors. A gate ofeach of the current controlling transistors is connected to one of theswitching transistors 24 a to 24 c, a source thereof is connected to oneof the current supplying lines 23 a to 23 c, and a drain thereof isconnected to one of EL elements 26 a to 26 c. 27 a to 27 c denotecondensers for holding the voltage applied to gates of the currentsupplying lines 25 a to 25 c. However, the condensers 27 a to 27 c maybe omitted.

The case shown in FIG. 13A is of using n-channel transistors for theswitching transistors 24 a to 24 c and p-channel transistors for thecurrent controlling transistors 25 a to 25 c. As shown in FIG. 13B, itis also possible to use p-channel transistors for switching transistors28 a to 28 c and n-channel transistors for current controllingtransistors 29 a to 29 c in each of a pixel (RED) 30 a, a pixel (GREEN)30 b, and a pixel (BLUE) 30 c.

FIGS. 13A and 13B show a case in which two transistors are provided inone pixel. However, the number of transistors may be more than two(typically, three to six). Any combination of n-channel transistors andp-channel transistors may be employed also when more than twotransistors are provided in each pixel.

In this embodiment, the EL element 26 a is a red light emitting ELelement and the EL element 26 c is a blue light emitting EL element, andboth of them use triplet compounds for their light emitting layers. TheEL element 26 b is a green light emitting EL element and a singletcompound is used for its light emitting layer.

By choosing between a triplet compound and a singlet compound in thisway, the El elements 26 a to 26 c can have the same operation voltage(10 V or less, preferably 3 to 10V). Thus the power supply required inthe light emitting device can uniformly be set to, for example, 3 V or 5V, to make the circuit design simpler.

The structure of this embodiment may be combined with any of thestructures of Embodiments 1 and 2.

Embodiment 4

This embodiment describes a case in which n-channel transistors are usedfor all of transistors that constitute a pixel portion and a drivingcircuit. The n-channel transistors are fabricated in accordance withEmbodiment 1, and explanations thereof are omitted.

The sectional structure of a light emitting device according to thisembodiment is shown in FIG. 14. The basic structure thereof is the sameas the sectional structure of FIG. 5B which is described inEmbodiment 1. Therefore only differences are picked up and explainedhere.

In this embodiment, an n-channel transistor 1201 is provided instead ofthe p-channel transistor 602 of FIG. 5B and a current controllingtransistor 1202 that is an n-channel transistor is provided in place ofthe current controlling transistor 604.

A wiring line 266 connected to a drain of the current controllingtransistor 1202 functions as a cathode of an EL element. Formed on thewiring line are an EL layer 1203, an anode 1204 formed of an oxideconductive film, and a passivation film 1205. The wiring line 266 isdesirably formed from a metal film containing an element belonging toGroup 1 or 2 in the periodic table. If not at least a surface of thewiring line 266 that is in contact with the EL layer 1203 is formed of ametal film containing an element belonging to Group 1 or 2 in theperiodic table.

The n-channel transistors used in this embodiment may be all enhancementtype transistors or depression type transistors. Alternatively,enhancement type transistors and depression type transistors may be usedin combination.

Now, the circuit structure of pixels is shown in FIG. 15. For the partsdenoted by the same reference symbols as those in FIGS. 13A and 13B,refer to explanations of FIGS. 13A and 13B.

As shown in FIG. 15, the switching transistors 24 a to 24 c and thecurrent controlling transistors 36 a to 36 c provided in a pixel (RED)35 a, a pixel (GREEN) 35 b, and a pixel (BLUE) 35 c, respectively, areall n-channel transistors.

According to the structure of this embodiment, the photolithography stepfor forming the p-channel transistors and the photolithography step forforming the pixel electrodes (anodes) in the process of manufacturing alight emitting device of Embodiment 1 corresponding to thephotolithography step for forming cathodes in this embodiment areeliminated. Therefore the manufacture process can be simplified evenmore.

The structure of this embodiment may be combined with any of thestructures of Embodiments 1 through 3.

Embodiment 5

This embodiment describes a case in which p-channel transistors are usedfor all of transistors that constitute a pixel portion and a drivingcircuit. The sectional structure of a light emitting device according tothis embodiment is shown in FIG. 16. For the parts denoted by the samereference symbols as those in FIG. 5B, refer to explanations ofEmbodiment 1.

In this embodiment, the driving circuit is composed of a PMOS circuitthat has a p-channel transistor 1401 and a p-channel transistor 1402whereas the pixel portion has a switching transistor 1403 that is ap-channel transistor and a current controlling transistor 1404 that is ap-channel transistor. An active layer of the p-channel transistor 1401includes a source region 41, a drain region 42, LDD regions 43 a and 43b, and a channel formation region 44. The p-channel transistor 1402, theswitching transistor 1403, and the current controlling transistor 1404have the same active layer structure as the p-channel transistor 1401.

Now, a process of manufacturing a p-channel transistor in accordancewith this embodiment will be described with reference to FIGS. 17A to17C. First, the manufacture process of Embodiment 1 are finished upthrough the step of FIG. 3B.

Next, electrodes 212 to 216 are formed from a second conductive filmusing resists 211 a to 211 e. The resists 211 a to 211 e and theelectrodes 212 to 216 formed of the second conductive film are then usedas masks to dope a semiconductor film with an element belonging to Group13 in the periodic table (boron, in this embodiment). As a result,regions 301 to 309 containing boron in a concentration of 1×10²⁰ to1×10²¹ atoms/cm³ (hereinafter referred to as p type impurity regions(a)) are formed (FIG. 17A).

The electrodes 212 to 216 formed of the second conductive film are thenetched using the resists 211 a to 211 e under the same etchingconditions as those in FIG. 3E to form second gate electrodes 310 to 314(FIG. 17B).

Next, the resists 211 a to 211 e and the second gate electrodes 310 to314 are used as masks to etch a first conductive film 209 under the sameetching conditions as those in FIG. 3D to form first gate electrodes 315to 319.

The resists 211 a to 211 e and the second gate electrodes 310 to 314 arethen used as masks to dope the semiconductor film with an elementbelonging to Group 13 in the periodic table (boron, in this embodiment).As a result, regions 320 to 329 containing boron in a concentration of1×10¹⁶ to 1×10¹⁹ atoms/cm³, typically 1×10¹⁷ to 1×10¹⁸ atoms/cm³(hereinafter referred to as p type impurity regions (b)) are formed(FIG. 17C).

The subsequent steps are the same as the step of FIG. 4C and thefollowing steps thereof in Embodiment 1. A light emitting devicestructured as shown in FIG. 16 is manufactured through the aboveprocess.

The p-channel transistors used in this embodiment may be all enhancementtype transistors or depression type transistors. Alternatively,enhancement type transistors and depression type transistors may be usedin combination.

The circuit structure of pixels is shown in FIG. 18. For the partsdenoted by the same reference symbols as those in FIGS. 13A and 13B,refer to explanations of FIGS. 13A and 13B.

As shown in FIG. 18, switching transistors 51 a to 51 c and currentcontrolling transistors 52 a to 52 c provided in a pixel (RED) 50 a, apixel (GREEN) 50 b, and a pixel (BLUE) 50 c, respectively, are allp-channel transistors.

According to the structure of this embodiment, one photolithography stepin the process of manufacturing a light emitting device of Embodiment 1is omitted. Therefore the manufacture process is more simplified thanEmbodiment 1.

The structure of this embodiment may be combined with any of thestructures of Embodiments 1 through 4.

Embodiment 6

An active matrix light emitting device of the present invention can alsoemploy an MOS (metal oxide semiconductor) transistor for a semiconductorelement. In this case, a MOS transistor formed on a semiconductorsubstrate (typically a silicon wafer) by a known method is used.

The structure of this embodiment, except for the semiconductor element,may be combined with any of the structures of Embodiments 1 through 5.

Embodiment 7

Embodiment 1 shows in FIGS. 6A and 6B the light emitting device withbuilt-in driving circuit as an example of the light emitting device inwhich a pixel portion and a driving circuit are integrally formed on thesame insulator. However, it is also possible to use an external IC(integrated circuit) for the driving circuit. In this case, thestructure thereof is as shown in FIG. 19A.

In a module shown in FIG. 19A, an FPC 63 is attached to an active matrixsubstrate 60 (including a pixel portion 61 and wiring lines 62 a and 62b), and a printed wiring board 64 is attached to the substrate throughthe FPC 63. A functional block diagram of the printed wiring board 64 isshown in FIG. 19B.

As shown in FIG. 19B, the printed wiring board 64 is provided with an ICfunctioning as at least I/O ports (also called input or output units) 65and 68, a source side driving circuit 66, and a gate side drivingcircuit 67.

A module in which an FPC is attached to an active matrix substrate witha pixel portion formed thereon and a printed wiring board functioning asa driving circuit is attached to the substrate through the FPC, as inthe module above, is specially called a light emitting module withexternal driving circuit in this specification.

In a module shown in FIG. 20A, an FPC 74 is attached to a light emittingdevice with built-in driving circuit 70 (including a pixel portion 71, asource side driving circuit 72, a gate side driving circuit 73, andwiring lines 72 a and 73 a), and a printed wiring board 75 is attachedto the light emitting device with built-in driving circuit 70 throughthe FPC 74. A functional block diagram of the printed wiring board 75 isshown in FIG. 20B.

As shown in FIG. 20B, the printed wiring board 75 is provided with an ICfunctioning as at least I/O ports 76 and 79 and a controlling unit 77.Although a memory unit 78 is provided here, it is not always necessary.The controlling unit 77 has a function of controlling the drivingcircuits and correcting video data.

A module in which a printed wiring board having a function as acontroller is attached to a light emitting device with built-in drivingcircuit with the driving circuit and a pixel portion formed on asubstrate, as in the module above is specially called a light emittingmodule with external controller in this specification.

Embodiment 8

The light-emitting device (including the module at the state of which isshown in Embodiment 9) formed by implementing this invention may bebuilt in various electrical appliances and thereof pixel portion is usedas a image display portion. As electrical appliances of this invention,there are a video camera, a digital camera, a goggle type display (headmounted display), a navigation system, an audio apparatus, a note typepersonal computer, a game apparatus, a portable information terminal(such as a mobile computer, a portable telephone, a portable gameapparatus or an electronic book), and an image playback device with arecording medium. Specific examples of the electronic equipment areshown in FIGS. 21 and 22.

FIG. 21A shows a display and includes a casing 2001, a supporting base2002 and a display portion 2003. The light-emitting device of thisinvention may be used for the display portion 2003. When using thelight-emitting device having the EL element in the display portion 2003,since the EL element is a self-light emitting type backlight is notnecessary and the display portion may be made thin.

FIG. 21B shows a video camera, which contains a main body 2101, adisplay portion 2102, a sound input portion 2103, operation switches2104, a battery 2105, and an image receiving portion 2106. Thelight-emitting device of this invention can be applied to the displayportion 2102.

FIG. 21C shows a digital camera, which contains a main body 2201, adisplay portion 2202, an eye contact portion 2203, and operationswitches 2204. The light emitting-device and the liquid crystal displaydevice of this invention can be applied to the display portion 2202.

FIG. 21D shows an image playback device equipped with a recording medium(specifically, a DVD playback device), which contains a main body 2301,a recording medium (such as a CD, LD or DVD) 2302, operation switches2303, a display portion (a) 2304, a display portion (b) 2305. Thedisplay portion (a) is mainly used for displaying image information. Thedisplay portion (b) 2305 is mainly used for displaying characterinformation. The light-emitting device of this invention can be appliedto the display portion (a) and the display portion (b). Note that, theimage playback device equipped with the recording medium includesdevices such as CD playback device, and game machines.

FIG. 21E shows a portable (mobile) computer, which contains a main body2401, a display portion 2402, an image receiving portion 2403, operationswitches 2404 and a memory slot 2405. The light-emitting device of thisinvention can be applied to the display portion 2402. This portablecomputer may record information to a recording medium that hasaccumulated flash memory or involatile memory, and playback suchinformation.

FIG. 21F shows a personal computer, which contains a main body 2501, acasing 2502, a display portion 2503, and a keyboard 2504. Thelight-emitting device of this invention can be applied to the displayportion 2503.

The above electronic appliances more often display information sentthrough electron communication circuits such as Internet or the CATV(cable television), and especially image information display isincreasing. When using the Light-emitting device having the EL elementin the display portion, since the response speed of the EL element isextremely fast, it becomes possible to display pictures without delay.

Further, since the light emitting portion of the light-emitting deviceconsumes power, it is preferable to display information so that thelight emitting portion is as small as possible. Therefore, when usingthe light-emitting device in the display portion where characterinformation is mainly shown in the portable information terminal,especially in a portable phone or an audio apparatus, it is preferableto drive so that the character information is formed of a light emittingportion with the non-light emitting portion as a background.

Here, FIG. 22A shows a portable telephone, which contains a main body2601, a sound output portion 2602, a sound input portion 2603, a displayportion 2604, an operation switch 2605 and an antenna 2606. Thelight-emitting device of this present invention can be applied to thedisplay portion 2604. Note that, when using the light-emitting device tothe display portion 2604, the consumption power of the portabletelephone may be suppressed by displaying white letters in thebackground of the black color.

FIG. 22B shows also a portable telephone, but it is a folding twice typedifferent from that of FIG. 22A, and contains a main body 2611, a soundoutput portion 2612, a sound input portion 2613, a display portion (a)2614, a display portion (b) 2615 and an antenna 2616. The operationswitch is not adhered to this type portable telephone, but its functionis provided to the portable telephone by displaying a characterinformation shown in FIGS. 22C, 22D and 22E by either of the displayportion (a) or (b). Further, another display portion displays mainly theimage information. The light-emitting device of the present inventioncan be used as the display portion (a) 2614 or a display portion (b)2615.

In the case of the portable telephone shown in FIG. 22B, thelight-emitting device used in the display portion 2604 is incorporatedwith a sensor by a CMOS circuit (a CMOS sensor), and may be used as anauthentication system terminal for authenticating the user by readingthe fingerprints or the hand of the user. Further, light emission may beperformed by taking into consideration the brightness (illumination) ofoutside and making information display at a contrast that is alreadyset.

Further, the low power consumption may be attained by decreasing thebrightness when using the operating switch 2605 and increasing thebrightness when the use of the operation switch is finished. Further,the brightness of the display portion 2604 is increased when a call isreceived, and low power consumption is attained by decreasing thebrightness during a telephone conversation. Further, when using thetelephone continuously, by making it have a function so that display isturned off by time control unless it is reset, low power consumption isrealized. It should be noted that this control may be operated by hand.

Further, FIG. 22F shows an audio reproduction devices, concretely a caraudio which contains a main body 2621, a display portion 2622, andoperation switches 2623 and 2624. The light-emitting device of thisinvention can be applied to the display portion 2622. Further, in thisembodiment, a car mounted audio (car audio) is shown, but it may be usedin a portable type or domestic type audio (audio component). Note that,when using a light-emitting device in the display portion 2622, bydisplaying white characters in a black background, power consumption maybe suppressed. It is especially effective for the portable type audioreproduction device.

In the case of the portable type electronic apparatuses shown in thisembodiment, the sensor portion is provided to perceive the externallight and the function to lower the brightness of display when it isused in the dark area as a method to lower the power consumption.

As in the above, the applicable range of this invention is extremelywide, and may be used for various electrical equipment. Further, theelectrical equipment of this embodiment may use the electronic devicecontaining any of the structures of Embodiments 1 to 8.

Embodiment 9

Embodiment 1 describes a case where the transistors are top gatetransistors. However, the transistor structure of the present inventionis not limited thereto and bottom gate transistors (typically reversestagger transistors) may also be used in carrying, out the presentinvention as shown in FIGS. 23A and 23B. The reverse stagger transistorsmay be formed by any method.

FIG. 23A is a top view of an EL module formed in manufacture of a lightemitting device that uses bottom gate transistors. A source side drivingcircuit 3001, a gate side driving circuit 3002, and a pixel portion 3003are formed therein. FIG. 23B shows in section a region a 3004 of thepixel portion 3003. The sectional view is obtained by cutting the lightemitting device along the line x-x′ in FIG. 23A.

FIG. 23B illustrates only a current controlling transistor out oftransistors that constitute a pixel transistor. Reference symbol 3011denotes a substrate and 3012 denotes an insulating film to serve as abase (hereinafter referred to as a base film). A transparent substrateis used for the substrate 3011, typically, a glass substrate, a quartzsubstrate, a glass ceramic substrate, or a crystallized glass substrate.However, the one that can withstand the highest process temperatureduring the manufacture process has to be chosen.

The base film 3012 is effective especially when a substrate containing amovable ion or a conductive substrate is used. If a quartz substrate isused, the base film may be omitted. An insulating film containingsilicon is used for the base film 3012. The term insulating filmcontaining silicon herein refers to an insulating film containing oxygenor nitrogen in a given ratio to the content of silicon, specifically, asilicon oxide film, a silicon nitride film, or a silicon oxynitride film(SiOxNy: x and v are arbitrary integers).

Reference symbol 3013 denotes a current controlling transistor that is ap-channel transistor. When an EL emits light toward the top face of thesubstrate (the face on which transistors and an EL layer are formed) asshown in this embodiment, it is desirable to use n-channel transistorsfor a switching transistor and a current controlling transistor as well.However, the present invention is not limited to thereto. The switchingtransistor may be an n-channel transistor or a p-channel transistor andthe same applies to the current controlling transistor.

The current controlling transistor 3013 is composed of an active layer,a gate insulating film 3017, a gate electrode 3018, a first interlayerinsulating film 3019, a source wiring line 3020, and a drain wiring line3021. The active layer includes a source region 3014, a drain region3015, and a channel formation region 3016. The current controllingtransistor 3013 in this embodiment is an n-channel transistor.

The switching transistor has a drain region connected to the gateelectrode 3018 of the current controlling transistor 3013. The gateelectrode 3018 of the current controlling transistor 3013 iselectrically connected to the drain region (not shown) of the switchingtransistor through a drain wiring line (not shown), to be exact. Thegate electrode 3018 has a single gate structure but may take amulti-gate structure. The source wiring line 3020 of the currentcontrolling transistor 3013 is connected to a current supplying line(not shown).

The current controlling transistor 3013 is an element for controllingthe amount of current supplied to the EL element, and a relatively largeamount of current flows through this transistor. Therefore, it ispreferable to design the current controlling transistor to have achannel width (W) wider than the channel width of the switchingtransistor. It is also preferable to design the current controllingtransistor to have a rather long channel length (L) in order to avoidexcessive current flow in the current controlling transistor 3013.Desirably, the length is set such that the current is 0.5 to 2 μA(preferably 1 to 1.5 μA) per pixel.

If the active layer (channel formation region, in particular) of thecurrent controlling transistor 3013 is formed thick (desirably 50 to 100nm more desirably 60 to 80 nm), degradation of the transistor can beslowed.

After the current controlling transistor 3013 is formed, the firstinterlayer insulating film 3019 and a second interlayer insulating film(not shown) are formed to form a pixel electrode 3023 that iselectrically connected to the current controlling transistor 3013. Inthis embodiment, the pixel electrode 3023 formed of a conductive filmfunctions as a cathode of the EL element.

Specifically, the pixel electrode is formed of an alloy film of aluminumand lithium. Any conductive film formed of an element belonging to Group1 or 2 in the periodic table or a conductive film doped with the Group 1(or 2) element can be used.

After the pixel electrode 3023 is formed, a third interlayer insulatingfilm 3024 is formed. The third interlayer insulating film 3024 serves asa so-called bank.

An EL layer 3025 is formed next. Shown in FIG. 23B in section is acolumn of pixels that be formed the same EL layer.

The EL layer in this embodiment uses Alq₃ for an electron injectionlayer, BCP for an electron transporting layer, and CBP doped with Ir(ppy)₃ for a light emitting layer. A hole transporting layer thereof isformed of α-NPD.

Next, an anode 3026 is formed from a transparent conductive film on theEL layer. The transparent conductive film used in this embodiment is aconductive film formed from a compound of indium oxide and tin oxide, ora compound of indium oxide and zinc oxide.

A passivation film is further formed on the anode from an insulatingmaterial to thereby complete an EL module having a reverse staggertransistor structure. The light emitting device manufactured inaccordance with this embodiment emits light in the direction indicatedby the arrow in FIG. 23B (toward the top face).

A reverse stagger transistor can be fabricated with a smaller number ofmanufacture steps than needed to fabricate a top gate transistor.Therefore it is very advantageous for cost down, which is one of theobjects of the present invention.

The structure of this embodiment may be combined freely with any of thestructures of Embodiments 1 through 8.

Embodiment 10

Described next in this embodiment is a case of introducing an SRAM to apixel portion. FIG. 24 shows an enlarged view of a pixel 3104. In FIG.24, reference symbol 3105 denotes a switching transistor. The switchingtransistor 3105 has a gate electrode connected to a gate signal line3106 that is one of gate signal lines (Gl to Gn) to which gate signalsare inputted. The switching transistor 3105 has a source region and adrain region one of which is connected to a source signal line 3107 thatis one of source signal lines (S1 to Sn) to which source signals areinputted, and the other of which is connected to an input side of anSRAM 3108. An output side of the SRAM 3108 is connected to a gateelectrode of a current controlling transistor 3109.

The current controlling transistor 3109 has a source region and a drainregion one of which is connected to a current supplying line 3110 thatis one of current supplying lines (V1 to Vn), and the other of which isconnected to an EL element 3111.

The EL element 3111 is composed of an anode, a cathode, and an EL layerinterposed between the anode and the cathode. When the anode isconnected to the source region or the drain region of the currentcontrolling transistor 3109, in other words when the anode is a pixelelectrode, the cathode serves as an opposite electrode. On the otherhand, when the cathode is connected to the source region or the drainregion of the current controlling transistor 3109, in other words, whenthe cathode is a pixel electrode, the anode serves as the oppositeelectrode.

The SRAM 3108 has two p-channel transistors and two n-channeltransistors. Source regions of the p-channel transistors are connectedto Vddh on the high voltage side whereas source regions of the n-channeltransistors are connected to Vss on the low voltage side. One p-channeltransistor and one n-channel transistor forms a pair, and one SRAM hastwo pairs of p-channel transistors and n-channel transistors.

A drain region of one p-channel transistor is connected to a drainregion of the n-channel transistor of the pair. A gate electrode of onep-channel transistor is connected to a gate electrode of the n-channeltransistor of the pair. Drain regions of the p-channel transistor andthe n-channel transistor of one pair are kept at the same level ofelectric potential as gate electrodes of the p-channel transistor andthe n-channel transistor of the other pair.

Drain regions of the p-channel transistor and the n-channel transistorof one pair receive input signals (Vin) and serve as the input side.Drain regions of the p-channel transistor and the n-channel transistorof the other pair send out output signals (Vout) and serve as the outputside.

The SRAM is designed to hold Vin and output Vout that is a signalobtained by inverting Vin. When Vin is Hi, Vout is a Lo signalcorresponding to Vss. When Vin is Lo, Vout is a Hi signal correspondingto Vddh.

In the case where one SRAM is provided in the pixel 3104 as shown inthis embodiment, a still image can be displayed while stopping theoperation of most of the external circuit because the memory data in thepixel is kept. This makes it possible to reduce power consumption. Onepixel may have a plurality of SRAMs. A plurality of data can be heldwhen plural SRAMs are provided in one pixel, making gray scale displayby time gray scale possible.

The structure of this embodiment may be combined freely with any of thestructures of Embodiments 1 through 9.

By carrying out the present invention, the luminance of light emittedfrom EL elements formed on the same substrate can readily be equalizedand a low power consumption light emitting device that can emit light ofhigh luminance with a low voltage can be obtained. Also, a low powerconsumption electric machine can be provided when this light emittingdevice is used in a display portion thereof.

1. A light emitting device comprising: a substrate; a first EL elementover the substrate; and a second EL element over the substrate, whereinthe first EL element comprises a first EL layer comprising a tripletcompound, wherein the second EL element comprises a second EL layercomprising a singlet compound, and wherein at least one of the first andsecond EL layers comprises a plurality of hole transporting layers.
 2. Alight emitting device according to claim 1, wherein the holetransporting layer includes at least a layer containing MTDATA and alayer containing α-NPD.
 3. A light emitting device according to claim 2,wherein the layer containing α-NPD is sandwiched between the lightemitting layer and the layer containing MTDATA.
 4. A light emittingdevice according to claim 1, wherein the triplet compound includes analuminoquinolilate complex.
 5. A light emitting device comprising: afirst substrate; a second substrate; a flexible printed circuit; a pixelportion including a first thin film transistor over the first substrate;a driver circuit including a second thin film transistor over the secondsubstrate; wherein the pixel portion comprises a first EL element and asecond EL element, wherein the first EL element comprises a first ELlayer comprising a triplet compound, wherein the second EL elementcomprises a second EL layer comprising a singlet compound, wherein atleast one of the first and second EL layers comprises a plurality ofhole transporting layers, and wherein the first substrate and the secondsubstrate are connected through the flexible printed circuit.
 6. A lightemitting device according to claim 5, wherein the hole transportinglayer includes at least a layer containing MTDATA and a layer containingα-NPD.
 7. A light emitting device according to claim 6, wherein thelayer containing α-NPD is sandwiched between the light emitting layerand the layer containing MTDATA.
 8. A light emitting device according toclaim 5, wherein the triplet compound includes an aluminoquinolilatecomplex.
 9. A light emitting device comprising: a substrate; a pixelportion including a first thin film transistor over the substrate; adriver circuit including a second thin film transistor over thesubstrate; wherein the pixel portion comprises a first EL element and asecond EL element, wherein the first EL element comprises a first ELlayer comprising a triplet compound, wherein the second EL elementcomprises a second EL layer comprising a singlet compound, and whereinat least one of the first and second EL layers comprises a plurality ofhole transporting layers.
 10. A light emitting device according to claim9, wherein the hole transporting layer includes at least a layercontaining MTDATA and a layer containing α-NPD.
 11. A light emittingdevice according to claim 10, wherein the layer containing α-NPD issandwiched between the light emitting layer and the layer containingMTDATA.
 12. A light emitting device according to claim 9, wherein thetriplet compound includes an aluminoquinolilate complex.
 13. An imageplayback device comprising: a main body; a display portion; an operationswitch; a substrate; a first EL element over the substrate; and a secondEL element over the substrate, wherein the first EL element comprises afirst EL layer comprising a triplet compound, wherein the second ELelement comprises a second EL layer comprising a singlet compound, andwherein at least one of the first and second EL layers comprises aplurality of hole transporting layers.
 14. An image playback deviceaccording to claim 13, wherein the hole transporting layer includes atleast a layer containing MTDATA and a layer containing α-NPD.
 15. Animage playback device according to claim 14, wherein the layercontaining α-NPD is sandwiched between the light emitting layer and thelayer containing MTDATA.
 16. An image playback device according to claim13, wherein the triplet compound includes an aluminoquinolilate complex.17. An image playback device according to claim 13, wherein the imageplayback device is one selected from the group consisting of a DVDplayback device, a CD playback device, and a game machine.